A torque converter is a device for transmitting torque from an engine to a transmission by fluid. The torque converter mainly includes: a front cover to which the torque from the engine is inputted; and components for forming a torus, i.e., an impeller, a turbine and a stator. Further, the torque converter includes a lock-up clutch device disposed axially between the front cover and the turbine.
The lock-up clutch device is a device for directly transmitting torque from the front cover to the turbine. As a type of the lock-up clutch device, a multi-plate type device has been provided that includes a plurality of clutch plates for increasing the capacity of transmitting torque. Here, the clutch plates are disposed on a front cover side (an input side) and a turbine side (an output side). Further, the clutch plates are configured to be press-contacted to each other by a piston configured to be actuated by hydraulic pressure. Accordingly, a clutch-on (torque transmission) state is produced.
The clutch plates are composed of: a plate to which friction members are fixed; and a plate to which no friction member is fixed. In general, the plate to which the friction member is fixed includes an annular core plate and friction members fixed to the both surfaces of the core plate. Further, as described in PTL 1, for lubrication and cooling and for reducing drag torque, a plurality of grooves are formed on the friction surface of each friction member so as to be extending from the inner periphery to the outer periphery.